Autorotational question

[Shenanigan]

Does anyone know what specific effects an increase or decrease in Rotor RPM during a descent has on the touchdown at the end?

Obviously it is known that if a full down collective steady state autorotation results in a increased rotor RPM than there will be an increase in rate of descent. This can be compensated for by increasing collective thus reducing rate of descent and increasing glide however at the touchdown point there will be a negative effect as there is now a smaller margin of collective left for cushion.

So if you found yourself in the situation where collective is needed to maintain 100% RPMR, and from my experience it is not uncommon is it more beneficial to let it rise some (10% or so) and convert that energy at the bottom? Or to use than energy during the descent despite having less energy for the cushion?


Disclaimer

I am not a student looking for an explanation of how to perform an autorotation. I think we can all appreciate that if you follow the parameters of RPM and airspeed set forth by the manufacturer you will achieve a successful autorotation. It is an aerodynamics question asked so i can better understand the relationship between rotor rpm vs angle of attack and why certain combinations can result in more lift for an equivalent torque as seen in variable rotor systems. And what these effects have in autorotation.

I am not talking about overspeeding the rotor until it is damaged. While I'm sure it is theoretically possible in autorotation to keep increasing the RPM until it falls off it has been my experience that the rotor RPM will stabilize even if slightly above 100% and the place where it stabilizes will normally be in the acceptable range except in extreme cases of DA and GWT. For this question assume the increase in rpm is within the acceptable range of the rotor.

I am not talking about practice autos where you may want to have a tighter parameter to reduce long term damage. I'm talking about an actual emergency where survival is a higher priority than long term durability of components.


I'm sure we have all heard from someone at some point in time that they believe that a high Rotor RPM is beneficial when it comes time for a touchdown. So is it more beneficial to arrive at your cushion with more potential energy but a greater rate of descent or to arrive with a lower rate of descent but less potential energy for cushion? Why?

[Widewoodenwingswork]

I had this discussion with another pilot not long ago.

This is purely my opinion but if a machine let me down and my chances of getting the machine out unscathed weren't great, I'd have absolutely no hesitation in putting the needles through the roof in the flare to give me a little more at the bottom as opposed to extending the flare and keeping the needles in the green.

If my chances looked favourable and the wind was on my nose, I'd do exactly as trained and trust that it was enough, but in saying that, an overspeed inspection is quite cheap compared to a heavy landing inspection (which usually involves the blades striking the tailboom).

The RFM is written with my best interests in mind so I'd be following that for the bulk of the auto and using my judgement where I felt necessary.

At the end of the day, I've always been told that if you can get yourself to the flare, you're a good chance of getting out alive. I wouldn't be too concerned about an extra 4 or 5 percent or 50 or 100fpm.

I'd be interested in hearing others opinions!

[EN48]

This is purely my opinion but if a machine let me down and my chances of
getting the machine out unscathed weren't great, I'd have absolutely no
hesitation in putting the needles through the roof in the flare to give me a
little more at the bottom as opposed to extending the flare and keeping the
needles in the green.

Strongly agree. Once the engine quits, the insurance company owns it. The helicopter is expendable. Do whatever you must do to live to fly another day. As my grizzled instructor is fond of saying, "I have never heard of anyone dying from a rotor overspeed."

[Shenanigan]

Thanks for the replies. I've certainly heard those sentiments many times but I've also heard the opposite that the resulting increase in rate of descent by a build over 100 must be controlled with collective or the aircraft becomes unsafe. I've also heard those that think a 90%ish rotor and the decrease in rate of descent is more beneficial.

Seems to me it would be the same difference as long as you actually used that potential energy before touchdown for cushion but my understanding of aerodynamics is not sufficient enough to say for sure.

[hueyracer]

It is (theoretically) possible that the helicopter reaches such a high rate of D, that it becomes very difficult to flare it to a safe touch down speed-especially in small openings....as we all know, the engine(s) are likely to quit not at the final of a long runway, but more or less when flying over areas where we do not want to land-not to say autorotate into...

Overspeeding shouldn´t be an option-and that you never observed a real "overspeeding" indicates that you never experienced a "engine off"-autorotation in a fully loaded helicopter (as we did in Maintenance Test Pilot Course)......

The R-rpm IS going to overshoot.....if you do not recover.
As the FM already takes into account a higher R-rpm for autorotation, one shouldn´t play around with his luck....as he will need it at the final touchdown phase of the AR....

[Shenanigan]

Hueyracer. I have no idea what you're trying to say here. The last paragraph is particularly cryptic to me.

Understood that at GWs above what the aircraft is rigged for the RPM increase could exceed limits but as stated in the OP that is not the scenario I'm asking about.

[hueyracer]

Just keep in mind, that there is a big difference in autorotation between a High inertia rotor system and a low inertia rotor system (i.g. Bell 205 vs R22).

While you still have lots of "power" available with low rpm in one system, you will find you´re running out of power in the other system-BEFORE the skids touch the ground.....(even in "normal loading conditions" without heavy weights)....

What can happen in an overspeed?
Main rotor head and transmission can shear, blades could delaminate-when you´re operating outside the given parameters of the flight manual.....

[[email protected]]

You can utilise the extra energy from the higher Nr in the flare by leading slightly with collective - ie containing the Nr with lever. The extra rotor thrust will help reduce your RoD and forward speed as you flare but still (if you do it with care) leave you 100% Nr for the cushion. The less speed and RoD you have when it comes to the cushion stage, the more survivable the landing as your cushion has less aircraft momentum to stop.

If you want more range in auto then droop the NR but make sure you recover it before you start the flare.

[HillerBee]

If you're RPM is already at the top of the green before you begin the flare it will definitely overspeed during the flare. So when you are in the steady state of an autorotation you should pull collective to keep it at a 100% and the ROD will be what you might expect as will as you're angle/distance. When you need a very agressive flare at the bottom, you might actually want your RPM lower to start the flare.

[Shawn Coyle]

I've never heard of a rotor in autorotation overspeeding to the point of immediate damage to itself or the dynamic components.
It's almost impossible to get a rotor system without power to get to such high rotor RPMs. Under power, possibly, but not in autorotation.

[the coyote]

Why not keep the RRPM where it needs to be given the situation you are in?

What does the flare do? Yes, it increases RRPM (very helpful), yes it decreases speed (even more important), but the MOST important thing is that it reduces your rate of descent. Most people banter around about 1800 fpm in stable auto. It is this ROD that will kill you when you impact the ground, unless you roll it up in a ball at high speed.

If you have a higher RRPM during a stabilised auto, you will have a higher ROD that you will then need to arrest at the bottom.

My goal in any auto is to arrive if possible with the correct mix of speed, ROD and RRPM for the environment I'm landing in. Make that happen with correct control. I cringe when I hear broadstroke statements like "I'd put the needles through the roof", because it implies a one size fits all approach that I don't advocate.

Twice I have been to the ground with the engine stopped. Once in a R22 and once in a H300. Both were uneventful touchdowns with no damage and RRPM within limits all the way. There simply wasn't the need, in those situations, to spin the rotor high, so why do it?

[Soave_Pilot]

Rotor RPM Overspeed

There was a fatal AW109 Koala accident here in Brazil w/ 7 aboard about a month ago, preliminary reports show the aircraft had engine failure in cruise flight and one of the blades was fond about 500 meters away from the crash site, it is strongly believed this blade came off during an overspeed from the transitioning (cruise/eng failure/flare to iniciate autorotation) as the Aw109 allows very little rotor overspeed.

I would think twice before letting my rotor overspeed

[Blackopsrider7]

I'm not a pilot, I'm a maintainer on a 412. After the first flight during a rotor track and balance procedure( if I may call it that), we set the auto-rotation speed to maintain 100% Nr or so. Yes, GW at the time of the auto will affect your Nr. I don't beleive a well maintained, well set up rotor is going to go past the dynamic parameters. Tha being said, when things go worng, they do go wrong, those damn swiss cheese holes!! I have seen close to 109% Nr though. HUMS shows a lot more than the naked eye!

[Widewoodenwingswork]

The above two posters are bang on the money. Overspeeding anything is bad. The coyote used his judgement and didn't need to overspeed the light rotor systems on two engine failures, he is to be commended. Can you give us anymore details coyote? How heavy, at what height and onto what surface.

My original post was simply saying that once the aircraft lets you down and a less than pleasant landing is likely, don't dismiss your judgement to keep a bunch of anonymous internet posters happy.

[lelebebbel]

We practice autorotations with the needles in the green and a "normal" ROD. Why would anyone want to deviate from that during a real emergency? The aircraft is going to react differently during the flare and touchdown if you rev the sh#t out of the rotors, and an autorotation after a power failure is not the right moment to become a test pilot.
Stick to what you trained, and you're less likely to stuff it up.

[EN48]

I dont read any poster here as indicating that one should deliberately overspeed the rotor. My interpretation is that extraordinary measures to keep the rotor in the green (on the high end) shouild be avoided if these otherwise compromise a safe outcome. There is a lot going on in a for real auto and rotor speed is usually not the highest priority (except to have enough). If you are good enough to do eveything just right, then do so. At the end of an auto, I am certainly not looking at the rotor tach. I'll take whatever speed my ears say is OK.

[Shenanigan]

Yes EN48 that's more of what I'm getting at.

As an example if in steady state my RPM is 107 with collective full down and that's in limits than will that have a positive effect on the decel, negative, or make no difference at all. On the opposite side, if I had put in 3 inches of collective and had a 95% RPM would that loss in potential energy and collective margin with reduction of rate of descent have an affect on the ability to cushion?

Both examples within limits. It's likely one method is better than another though the difference may be barely noticeable.

The collective against the full stop would require less concentration than an intermediate spot- (then check...then readjust to achieve exactly 100) at a time when concentration likely needs to be outside. So another variable is the benefit of simplicity against the possible inconsequential rise of RPM a few %. I think that thats what people are saying when they say they don't care a lot about the RPM in an actual emergency. If you just drop the collective hit the recommended airspeed and find an LZ than 99% of the time the RPM will take care of itself (by design, even if not at exactly 100%) by the time you need to decel.

[merlin_driver]

Your point seems to be simplicity of technique, but if you are flying a heli with a low inertia rotor, the actual difference in rate of descent may be small.
Still, regarding your example, and as someone pointed out before, you will probably not enter autorotation on a nice final to a runway. Therefore, if you want to increase/decrease range, to reach a specific point, you will have to use not only rate of descent, but also speed. Higher NR and lower speed to decrease range, vice versa to increase range, but then you have to adapt your flare technique to the lower/higher rate of descent.
Theoretically you may have a point, leaving the lever in a fixed position may be an "easier" technique, but then the increased rate of descent will force you to start the flare higher and/or be more agressive, probably something you don't usually train... Then you have a higher chance of hitting the ground with the tail, misjudging your height for collective application, etc.
In real life there are so many factors, there is not a "one size fits all" solution...

[topendtorque]

I've never heard of a rotor in autorotation overspeeding to the point of immediate damage to itself or the dynamic components.
It's almost impossible to get a rotor system without power to get to such high rotor RPMs. Under power, possibly, but not in autorotation.

The problem arises Shawn when people either deliberately set the auto RRPM high or don't bother to check it when it slowly creeps up, as they often do when things wear.

They then have an extraordinary high auto RPM setting that if not monitored will give horrendous spin up and of course far higher ROD. Not a pleasant sensation to be watching 'some' flying schools doing autos and hearing the rotors spin up to that blue note. Stretches the spindle bearings like nothing else.

For the newbies, in auto there is only one power source and that ain't changed in millions of years.

Faster you spin them rotors gadgets, more energy you use, faster you go down. Always set them up IAW the Maintenance Manual and check it every 100 hours.

That will also give you much better throttle correlation instead of having to fight it.

Reminds me of a blog I often read and sometimes contribute to, it's Tim Blair's and he's a very funny man who excercises extreme brevity. Just recently he coined a phrase "Hit the Rotors". One bloke reckoned his grandad "Hit The Rotors" at 99. Struck a chord with me anyway. He was referring to the stupid birds that fly into these even more stupid so called green energy gadgets - wind turbines. A few days before that he had a hilarious thread on correct punctuation.

So do it right an' you won't "Hit The Rotors" too early.

[[email protected]]

Shenanigan, 95% Nr should be used to increase range and then you should recover to 100% before starting the EOL flare.

If you start the flare with 95% and 3 inches of collective you won't increase the Nr very much and your Nr will decay faster once the flare is over, leaving you with a lot less to cushion.